Link to section on references when we use the term prior to defining it

Author: datagrok

src/doc/book/primitive-types.md

@@ -192,11 +192,13 @@ documentation][slice].
 # `str`
 
 Rust’s `str` type is the most primitive string type. As an [unsized type][dst],
-it’s not very useful by itself, but becomes useful when placed behind a reference,
-like [`&str`][strings]. As such, we’ll just leave it at that.
+it’s not very useful by itself, but becomes useful when placed behind a
+reference, like `&str`. We'll elaborate further when we cover
+[Strings][strings] and [references][].
 
 [dst]: unsized-types.html
 [strings]: strings.html
+[references]: references-and-borrowing.html
 
 You can find more documentation for `str` [in the standard library
 documentation][str].